[lt_dan]

Hi, I'm trying to run a transient simulation of an intake system for a four cylinder engine. I want to set the boundary conditions for the four outlets as pressures and I have been able to read transient pressure data from a table. However, I don't know how to simulate the opening and closing of the intake valve properly. My first thought was that the boundary type needed to change from a pressure outlet to a wall condition mid simulation although I'm not sure this is possible. I'm not that proficient with the script language and I wasn't able to get a conditional statement to work in the command editor thing. Any help would be great thanks!

[ghorrocks]

I assume the pressure you are trying to apply is the cylinder pressure. Maybe you could make the valve boundary a mass flow rate boundary, and calculate the mass flow rate based on the pressure difference across the valve from the manifold to the cylinder using the valve flow coefficients (which hopefully you know). You can easily set the mass flow rate to zero when the valve is shut this way.

By the way, you may be interested in my PhD thesis where I used CFX to model the inlet manifold and cylinder of an IC engine: https://opus.lib.uts.edu.au/bitstrea...14/01Front.pdf

[lt_dan]

That's correct, I have cylinder pressure data for the intake stroke from 1d sims. I was intending to have mass flow to each cylinder be an output of the simulation. Is it necessary to model the movement of the valves to do this? Or is there someway to modify the boundary condition type?

If it is more common to set mass flow boundary conditions at the outlets then perhaps I'm going about this incorrectly but regardless thanks for your help.

[ghorrocks]

In my suggestion the mass flow rate would be an output of the simulation. That is because the mass flow rate is a function of the inlet manifold pressure at the valve. The mass flow rate is not pre-determined.

If you are modelling a standard poppet valve engine and are only interested in the inlet manifold waves then you are not going to loose much by not modelling valve motion, and just applying a boundary condition on the valve curtain face at the fully open position.

Please note that my suggestion is just one way of doing it. What is best depends on what information you have to drive the simulation (you say you have cylinder pressure data).

A final comment - the cylinder pressure is coupled to the inlet manifold, so if you require more accuracy you should not assume a cylinder pressure but should model it. This adds considerably to the complexity of the simulation, and whether that complexity is worth it depends on what you are trying to achieve.

[lt_dan]

I was hoping to look at both the pressure waves through the manifold as well as the cylinder to cylinder variation of mass flow. I'm still a little unsure how the mass flow would be an output if I set the boundary conditions as mass flow derived from cylinder pressure? Previously I had just done steady state simulations to check mass flow variation with just a pressure drop. Unless I'm missing something, setting the outlet condition as a mass flow would mean all four cylinders would get equal mass flow (I'm using the same pressure trace for each cylinder).

Thank's a bunch for your help!

[ghorrocks]

Yes, you are missing something

The valve discharge coefficient is defined here: https://en.wikipedia.org/wiki/Discharge_coefficient

You can recast this as m(dot) = C(d).A.sqrt(2.rho.deltaP). You should know C(d) and A from the valve opening area, rho is the density at the boundary and deltaP is the difference between the pressure at the boundary and your function's cylinder pressure. The equation gives you m(dot) which you can apply as a mass flow rate at the inlet valve. So you are not specifying a fixed mass flow rate, you are specifying a variable mass flow rate which is a function of your inlet manifold pressure. When your inlet manifold pressure changes so will your m(dot).

Does that clarify it a bit?